The present invention relates generally to telephone circuits and, more particularly, to a subscriber circuit which can detect the user going off-hook during ringing even in the presence of high longitudinal current.
Subscriber line interface circuits (SLIC) are well-known in the prior art. These circuits, which serve to connect the subscriber to a telephone exchange, typically include a current detector which performs several functions including detection of the subscriber going off-hook during ringing (ring-trip). In particular, when the subscriber is being called, A.C. ringing signals are applied to the subscriber loop to operate the telephone ringer. When the subscriber goes off-hook, the resulting loop current is detected to interrupt the ringing signals on the loop.
In prior art interface circuits, the current detector can accurately detect the loop current even in the presence of high longitudinal currents. These longitudinal, or common-mode current arises from pair imbalance or the unequal coupling of a noise source into the wires of the pair. The current detector, however, can only detect the loop current if its direction is known. This shortcoming makes the circuit impractical during the ring mode since during this mode the current detector must detect A.C. loop current. If the detector is not in the proper mode; i.e., the direction of the ringing current is unknown, the circuit is unable to detect ring-trip in the presence of high longitudinal currents. Moreover, the polarity of the loop current is not easily determined since distributed capacitance on the line and the inductance of the ringer cause a phase difference between the line voltage and current waveforms. Therefore, there is a need to provide an improved current detector for a SLIC which is always in the proper directional mode to detect the user going off-hook during ringing.